It has been known that a nonwoven fabric with fine openings can be produced from fibers with a small fiber diameter. One of such methods is calendering a nonwoven fabric using heated rollers, wherein an opening size is decreased by reducing distances between fibers. When a nonwoven fabric is subjected to calendering, however, the thickness is reduced and the porosity of the nonwoven fabric is lowered. Accordingly, when calendering is performed under conditions to obtain a nonwoven fabric with sufficiently small openings, an intrinsic property of nonwoven fabrics of having voids therein is impaired. When heated rollers are used, fibers on the surfaces of a nonwoven fabric are thermally fusion-bonded, and thus the fibers may form a film or the pores may be squashed (for example, see Japanese Patent Application Laid-Open No. 8-246309). It has been pointed out that, when using a nonwoven fabric for light diffusion materials, liquid absorbers, filter materials, heat insulating materials and the like, if the nonwoven fabric has a low porosity, a reduction in diffusion performance, liquid retention performance, lifetime of filter material, heat insulation performance, or the like, is caused. Accordingly, a nonwoven fabric with some extent of high porosity and small uniform pore size has been desired.    [Patent Document 1] Japanese Patent Application Laid-open No. 8-246309